Autobiography is, first, a theory of individual development towards flourishing (aka well-being, happiness, success), using Mill's own life exemplar and warning; second, it's Mill's attempt to understand and account for himself on his own psychological and ethical principles. As a theory of development, it contains an ethology (an associationist account of the development of character out of circumstance) and an ideal of successful virtue and sensibility (Mill's wife Harriet, doing the rhetorical work for Mill that Cleanthes does for Hume in section 9 of Enquiry Concerning the Principles of Morals). Its central ideas are, first, progress (and it hints at parallels between individual and social development - Mill presents his intellectual life as gradual progress punctuated by revolution, for example); second, experiment and empiricism against intuition and the status quo.
As usually happens when I read Mill, I now want to drop what I was doing and read more. I have never read Principles of Political Economy, for example, and I could stand to reread Subjection of Women and Logic of the Moral Sciences.